BACKGROUND:Hammer Brothers are an elite force of Koopas armed with...hammers! They've appeared in many of Mario's adventures and are quite well known.
Because of the weapons they carry around with them, Hammer Brothers are among the more dangerous enemies Mario must face. They usually appear in pairs (hence the "brothers") and can be quite pesky unless armed with fireballs.
Although they are some of the most dangerous Koopas around, Hammer Brothers are fairly rare. Perhaps it takes a long time to train new Hammer Brothers, because if Bowser could get his whole army using these weapons, it might be big trouble for Mario and Luigi.

EVOLUTION:In the early years the Hammer Brothers changed appearances regularly. In the first Super Mario Brothers (and in SMB LL) they were mostly green, with orange limbs, and they threw regular hammers. In addition to going to larger, double-sided hammers in Super Mario Brothers 3, they also became completely orange-skinned, with a black shell and helmet. Their faces were a bit hard to see, but this was quite close to the character art made at the time.
In Super Mario RPG, Hammer Brothers went back to the look from SMB3, and this was the last time they were seen for some time. They returned in Paper Mario (not pictured) where they had dark blue armour and finally in Mario & Luigi: Superstar Saga,
they have green armour and angry eyes - these are probably the closest to the original artwork that these guys have gotten.
Finally, in Paper Mario: The Thousand Year Door, they look similar to the ones in Paper Mario with their blue helmets, these ones are very cartoony looking.
Hammer Bros. have since appeared in Super Paper Mario and the New Super Mario Bros. games with fairly standard appearances in each.